I was fasting.
I fasted for three days last year.
He is on a fast.
She is going to try a fast next week.
I'm honestly not sure if there's a specific verb that should be used with "fast" as a noun (by this I mean I don't know if one "goes on a fast", "does a fast" etc), as it's normally simply used as a verb on its own.
Student or Learner